U.S. Pat. No. 4,969,737 issued on Nov. 13, 1990 to Thomas W. Dey (co-inventor) discloses a Foucault knife-edge test for an objective or imaging device. The entire disclosure of this patent is incorporated herein by reference.
As disclosed therein and as shown in FIG. 1, optical assembly 10 demonstrates the basic principles of the Foucault knife-edge test. The assembly 10 includes a conventional imaging device, i.e. lens 12, comprising a pair of optical surfaces 14 and 16, radiation source 18, collector lens 20, and conventional photodetector 22 comprising the human eye. The components of assembly 10 are aligned to reference axis 24.
For optical assembly 10, one may employ the knife-edge test for qualitatively detecting (at eye/photodetector 22) the presence of transverse aberrations that may have been introduced into assembly 10 by lens optical surfaces 14 and 16. Accordingly, knife-edge 26 may be gradually introduced into assembly 10 (shown by way of the staggered arrows in FIG. 1), so that knife-edge 26 sequentially cuts and blocks the image of radiation source 18 at a plane of convergence 28. This action, in turn, removes source rays from their expected trajectories, so that a variable intensity pattern may be registered by the eye. Finally, a comparison of this intensity pattern with a theoretical intensity pattern for an ideal optical surface may become a qualitative measure of the presence of transverse aberrations introduced by optical surfaces 14 and 16.
Optical assembly 10 may be modified to obtain a quantitative interpretation of the Foucault knife-edge test. FIG. 2 shows the basic Foucault assembly 10 of FIG. 1, but modified to help realize quantitative interpretations of the knife-edge test. It is first noted that the eye has been replaced by a conventional photodetector 30. For example, photodetector 30 may comprise a matrix (m×n) array of charge coupled devices (CCD) where m is preferably from 64 to 1024, and n is preferably from 64 to 1024. The photodetector device 30 collects the radiation images by imaging device 12 under test, and provides, for each element in the matrix, a value proportional to the radiation intensity at that element. FIG. 2 shows that the outputs of photodetector 30 may be fed along line 32 to a conventional computing means 34.
Turning next to U.S. Pat. No. 5,020,905, issued on Jun. 4, 1991 to Thomas W. Dey, application of a Foucault knife-edge test to a segmented mirror is described. The entire disclosure of this patent is incorporated herein by reference. As disclosed therein and as shown in FIG. 3, segmented optic 38 includes a segmented mirror comprising two physically de-coupled, monolithic mirror sections 40 and 42. An individual and disjoint entrance pupil contribution by each of the physically de-coupled, monolithic mirror sections 40 and 42 aggregates in sum to form a common entrance pupil 36, i.e. entrance pupil 36 is developed over the entire surface of segmented optic 38.
The segmented optic 38 of FIG. 3, more particularly, may include an aluminized reflective coating on a Pyrex glass substrate. Here, segmented optic 38 has an overall diameter of approximately 125 mm, and a radius of curvature of approximately 2000 mm.
The Foucault testing of segmented optic 38 may proceed, with reference to assembly 10 of FIG. 1, mutatis mutandis, the required necessary changes being that of (1) replacing lens 12 of FIG. 1 with that of segmented optic 38 of FIG. 3, and (2) re-locating radiation source 18 to accommodate the reflective properties of mirror sections 40 and 42.
The Foucault testing of segmented optic 38 works by reconstructing, or emulating, an idealized monolithic mirror, by using Foucault determined data derived from sections 40 and 42, to align them into correspondence with the idealized monolithic mirror. Note that the Foucault determined data may be qualitative (for example when photodetector 22 of FIG. 1 includes the human eye). It is possible that the segmented optic 38 may induce an intensity pattern at the eye, in which the intensity pattern has inherent ambiguities, namely an ambiguity as to which of the two mirror sections 40 or 42 is indeed the source of an optical aberration. For this situation, one may employ the quantitative Foucault techniques described with respect to assembly 10 of FIG. 2.
Large segmented mirrors, for example segmented concave mirrors used as a primary mirror of an imaging telescope, are significantly misaligned in their initial deployment state. These segmented mirrors must be aligned to properly capture a light beam from an interferometer. Once aligned, the light beam from the interferometer may be used to interrogate (or test) the primary mirror at the mirror's center of curvature.
The segments of the primary mirror must be registered to extreme accuracy in order for the mirror to deliver image quality comparable to that of an equivalent monolithic mirror. Accordingly, the segments of the mirror are mechanically tip-tilted relative to each other, in order to achieve an ideal mirror configuration.
The present invention addresses a solution to the problem of how to align a large segmented mirror and achieve an accuracy sufficient for interrogation by an interferometer.